<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        *{margin:0;padding:0}
    .content{
        margin:50px;
        width:200px;
        height:400px;
        background: #333;
        position:relative;
    }
    .content::before{
        content:"";
        display: block;
        position: absolute;
        width:0;
        height:0;
        border:20px solid transparent;
        border-bottom-color:#333;
        top:-40px;
        z-index:100;
        left:50%;
        transform: translateX(-50%);
    }
    </style>
</head>
<body>
    <div class="content">

    </div>
</body>
</html>